The invention relates generally to optical fibers and more specifically to optical fibers with reduced cladding-mode losses.
Fiber Bragg gratings are used in various wavelength-dependent signal-processing applications, most notably in wavelength-division multiplexing (WDM) and dense wavelength-division multiplexing (DWDM) systems. In these systems, light of multiple wavelengths propagates through a transmission medium, typically an optical fiber. Because the wavelengths are spaced apart spectrally and do not interfere with each other, they represent separate communication channels that can be independently modulated to carry information. To select a particular channel, its wavelength must be extracted (i.e., demultiplexed) from the multiple-wavelength signal.
Fiber Bragg gratings are used to filter, reflect and/or demultiplex WDM signals. The grating is essentially the opposite of a bandpass filter, efficiently reflecting light within a narrow spectral band; as a result, the reflected band may be routed to another part of the system and/or analyzed to recover information encoded therein, or not used at all. A series of gratings may be used to extract multiple wavelengths from a signal for separate analysis or rerouting, or to confine the propagating signal to a single channel. The fiber Bragg grating is contained within an optical fiber, eliminating the need for separate bulk optical filter devices that add cost and complexity.
When the forward-propagating core and cladding modes reach the fiber Bragg grating, light at the Bragg wavelength is reflected along with some of the cladding modes (i.e., light propagating in the cladding of the fiber). The backward-propagating cladding modes couple with the forward-propagating core modes resulting in cladding-mode coupling loss (CMCL). CMCL degrades the performance of the Bragg grating, and thus the performance of the overall system.
The present invention is directed toward attenuating the power propagating in the cladding of an optical fiber, thereby reducing the amount of coupling between the cladding mode power reflected at the Bragg grating interface with the forward propagating core modes. In other words, the cladding-mode coupling loss is reduced.
Accordingly, in one aspect, the invention relates to a method of attenuating the cladding modes in an optical fiber in conjunction with a fiber Bragg grating. The method includes the step of providing an optical fiber having a core configured to transmit radiation to a fiber Bragg grating, a cladding surrounding the core, and within the cladding, an annular region of lossy material concentric with the core. Additionally, the method includes the step of propagating radiation through the core. The radiation consists of a core mode propagating through the core and a cladding mode propagating through the cladding near the core. The lossy region attenuates the cladding mode without substantially affecting the core mode, thereby reducing the attenuation of the core mode as a result of coupling with the cladding mode.
In one embodiment, the annular region of lossy material is slightly displaced radially from the core. The lossy material may be a titanium compound (e.g., titanium oxide) and/or rich in Ti3+. A portion of the core may define the fiber Bragg grating.
Another aspect of the present invention is directed to a fiber adapted to selectively affect at least one wavelength from a radiation signal propagating through the fiber. The fiber includes a core, a cladding, and a lossy region within the cladding. The core receives radiation, which propagates to a fiber Bragg grating. The fiber Bragg grating may be defined within the core and selectively affects at least one wavelength. The cladding surrounds the core as does the lossy region, which is annular in shape. Radiation propagating in the core consists of a core mode propagating through the core and a cladding mode propagating through the cladding near the core. The lossy region attenuates the cladding mode without substantially affecting the core mode, thereby limiting loss of the core mode as a result of coupling with the cladding mode.